Additional promotion for addons on sale

[WilliamVenner]

Feature

GmodStore customers are bargain hunters, especially for higher priced addons. We should highlight and promote addons that are currently on sale and help users to find addons on sale more easily.

1. On https://www.gmodstore.com/market/browse users should be able to filter for addons that are currently on sale (Filter by On Sale #1121)
2. On the front page, an "Addons On Sale" section should appear under "Newcomers", showing a random selection of addons that are currently on sale
3. Users should be notified when an addon on their wishlist goes on sale

Additional Information

No response

[TheodoorP]

Related to #1121